Green’s Dictionary of Slang

butter adj.2

also butta
[‘smooth as butter’]

(orig. US black/teen) a general term of approval, attractive, excellent etc.

[US]Mad mag. Apr. 32: Boo-yaaa!!! ’Cause Hooked on Ebonics is butter!
[US]Da Bomb [Internet] 5: Butter: Impressive.
[US]Source Oct. 22: Redman and Keith Murray come through with some butta lyrics.
[US]C. Eble (ed.) UNC-CH Campus Sl. Spring 2014 2: BUTTER — done with skill: ‘James-Michael’s dunk was butter’ .